1. Field of the Invention
The present invention relates to an image pickup apparatus for use in a digital still camera, a mobile phone camera, a Personal Digital Assistant (PDA) camera, an image inspection apparatus, an industrial camera used for automatic control, etc., which includes a detector (what is called image pickup device, such as CCD and CMOS) and an optical system. The present invention also relates to a method and an apparatus for manufacturing the image pickup apparatus.
2. Description of the Related Art
Recently, as in digital cameras, solid-state detectors, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been provided on imaging planes instead of films.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by a detector in the form of an electric signal. Such an apparatus is used in, for example, a digital still camera, a video camera, a digital video unit, a personal computer, a mobile phone, a PDA, an image inspection apparatus, an industrial camera used for automatic control, etc.
FIG. 38 is a schematic diagram illustrating the structure of a known image pickup apparatus and the state of ray bundles. Referring to FIG. 38, an image pickup apparatus 1 includes an optical system 2 and a detector 3, such as a CCD and a CMOS sensor. The optical system 2 includes object-side lenses 21 and 22, an aperture stop 23, and an imaging lens 24 arranged in that order from an object side (OBJS) toward the detector 3. In the image pickup apparatus 1, the best-focus plane coincides with the plane on which the detector 3 is disposed. FIGS. 39A to 39C show spot images formed on a light-receiving surface of the detector 3 included in the image pickup apparatus 1.
In such an image pickup apparatus, in order to achieve a large depth of field, a method has been suggested in which light is regularly blurred by a phase plate and is reconstructed by digital processing. On the other hand, an automatic exposure control system for a digital camera in which a filtering process using a transfer function is performed has also been suggested.
As a focusing method, a so-called hill-climbing autofocus (AF) method is known in which a focal position is determined by acquiring a peak value of contrast.
In known image pickup apparatuses, it is premised that a Point Spread Function (PSF) obtained when the above-described phase plate is placed in an optical system is constant. If the PSF varies, it becomes difficult to obtain an image with a large depth of field by convolution using a kernel.
In particular, in lens systems like zoom systems and autofocus (AF) systems, there is a large problem in adopting the above-mentioned structure because high precision is required in the optical design and costs are increased accordingly. More specifically, in known image pickup apparatuses, a suitable convolution operation cannot be performed and the optical system must be designed so as to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, to eliminate the aberrations, the complexity of the optical design is increased and the number of design steps, costs, and the lens size are increased.
For example, when an image obtained by shooting an object in a dark place is reconstructed by signal processing, noise is amplified at the same time. Therefore, in the optical system which uses both an optical unit and signal processing, that is, in which an optical wavefront modulation element, such as the above-described phase plate, is used and signal processing is performed, there is a problem that noise is amplified and the reconstructed image is influenced when an object is shot in a dark place.
In addition, when the aperture is stopped down to shoot a bright object, the phase modulation element is covered by the aperture stop and the phase variation is reduced. This affects the reconstructed image when the image reconstruction process is performed.
When the image is blurred on purpose as in the above-described optical system including the phase modulation element, the contrast is low. Therefore, it is difficult to obtain an in-focus state by the above-described focusing method.